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Hidden Food in the Coldest of Times the NUTRIENT ROLE of SEA ICE Copepods, Tiny Lipid-Rich Obtaining Sufficient Food Under Fig

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Hidden Food in the Coldest of Times the NUTRIENT ROLE of SEA ICE Copepods, Tiny Lipid-Rich Obtaining Sufficient Food Under Fig UNDERSTANDING ECOSYSTEM PROCESSES IN THE BERING SEA 2007–2013 Hidden Food in the Coldest of Times THE NUTRIENT ROLE OF SEA ICE Copepods, tiny lipid-rich obtaining sufficient food under Fig. 1 crustaceans in the Bering Sea, the ice to initiate feeding and are a favored meal of larval and reproduction. A second question juvenile Pollock. One copepod is “what is this food source”? One that dominates the zooplankton possibility was the layer of ice on the Bering Sea shelf, and shelf algae—a diverse community of areas around the Arctic Ocean, microscopic plants and animals is Calanus glacialis (Figure 1). that grow under the ice during We know that the abundance of spring (Figure 2)—rather than this species fluctuates between from the more usual phytoplank- years. Surprisingly, colder years, ton community in the water when ice cover is more extensive column beneath. and persists longer during spring, appear to favor growth of the How We Did It copepod population. Why is this? We collected zooplankton We set out to answer this ques- samples to see what the C. glacialis Fig. 2 tion during a cruise in late winter population was doing, whether of 2009 through early spring of the adult females were laying eggs 2010, when ice covered most of or not, and how much food was the Bering Sea shelf. in their guts. We determined the Since reproduction and identity of individual prey spe- growth of this copepod is con- cies in their guts from their DNA trolled by the availability of food, and quantified the amount of this we thought that they must be continued on page 2 The Big Picture The high feeding rates of the copepod Calanus glacialis that we observed during a cruise in late Blocks of sea ice turned winter and early spring of 2009/2010 could not have been sustained by the low levels of phyto- upside down to reveal plankton in the water column. This, and the presence of ice algae found in their guts, indicates that a thick layer of ice the copepods were obtaining their nutrition from ice algae. The higher feeding rates appeared to be algae. associated with warmer air temperatures, which are, in turn, associated with the release of ice algae into the water. Before this ice algae is diluted by dispersion into the water column below, it is likely that it provides a dense layer of food for the zooplankton. In years when when ice cover is more extensive and persists longer, there is an extended period of higher food availability for C. glacialis, compared with the brief ice-edge or water column phytoplankton bloom. This results in a longer period of population growth, resulting in greater abundance later during the spring. A NOVEL MOLECULAR APPROACH TO MEASURING IN SITU FEEDING RATES OF COPEPODS IN THE SOUTH EASTERN BERING SEA A component of the BEST-BSIERP Bering Sea Project, funded by the National Science Foundation and the North Pacific Research Board with in-kind support from participants. prey DNA to calculate consump- • C. glacialis eggs began to appear Fig. 4 tion (Figure 3). The amounts of in the water column during the phytoplankton chlorophyll in their cruise. At the same time we found guts provided an independent large amounts of phytoplankton measure of consumption (Figure 4). chlorophyll in the guts of adult We also measured the amount of female C. glacialis, indicating phytoplankton present in the water elevated feeding rates. column beneath the ice, and used DNA analysis to characterize the • The prey DNA in the guts of C. potential prey species present both glacialis was mostly from ice algal in the water column and in the ice species, indicating that ice algae algal community growing on the were an important source of food. underside of the ice. • Quantification of this prey DNA Estimated daily consumption by Calanus glacialis What we found: followed the same pattern of varia- adult females of the four diatom prey species, tion over time as the chlorophyll a based on the 18S DNA copy numbers in their • We found phytoplankton in guts, plotted against estimated phytoplankton extremely low concentrations pigments, indicating that DNA consumption, based on gut pigments. within the water column under can be used to provide a measure the ice, while a dense layer of ice of feeding rate on individual prey algae was at the base of the ice species. Why We Did It at most locations. Population growth of the domi- nant Arctic copepod C. glacialis Fig. 3 appears to be dependent upon seasonal ice cover and its associated ice algae during spring. Changes in the extent of this seasonal ice cover associated with climate change will adversely affect higher trophic levels that feed upon this key species. The Bering Sea Shelf is a region of rapid climate change. Knowledge of how key species respond to this change will help predict overall ecosystem response, and how important fisher- ies, as well as endangered species, may be affected. Edward Durbin, University of Rhode Island Graduate School of Oceanography The Bering Sea Project is a partnership between the North Pacific Research Board’s Bering Sea Integrated Ecosystem Research Program and the National Science Foundation’s Bering Ecosystem Study. www.nprb.org/beringseaproject Mean water column chlorophyll a, Calanus glacialis adult female gut pigments, abundance, egg abundance in the water, and estimated egg production rates between March 17 (Day 70) and March 31 (Day 90) 2010. A NOVEL MOLECULAR APPROACH TO MEASURING IN SITU FEEDING RATES OF COPEPODS IN THE SOUTH EASTERN BERING SEA A component of the BEST-BSIERP Bering Sea Project, funded by the National Science Foundation and the North Pacific Research Board with in-kind support from participants..
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